We report the observation of nonadiabatic excitations of single electrons in a quantum dot. Using a tunable-barrier single-electron pump, we have developed a way of reading out the excitation spectrum and level population of the dot by using the pump current as a probe. When the potential well is deformed at subnanosecond time scales, electrons are excited to higher levels. In the presence of a perpendicular magnetic field, the excited states follow a Fock-Darwin spectrum. Our experiments provide a simple model system to study nonadiabatic processes of quantum particles.
"However, since the devices investigated in this study were similar to those in , we used the highest field accessible with our experimental system, 14 T. Previous high-resolution measurements using sine wave drive could not be extended above f ≈ 350 MHz because the quantised plateau degraded with further increase in f . The general mechanism for this is not currently understood, although clear signatures of backtunneling due to non-adiabatic excitation at high f have been seen in some samples . To extend the pump operation range to higher frequencies, we developed a technique using an arbitrary waveform generator (Tektronix AWG7122B) to generate a custom V G1 (t) waveform . "
[Show abstract][Hide abstract] ABSTRACT: Electron pumps generate a macroscopic electric current by controlled manipulation of single electrons. Despite intensive research towards a quantum current standard over the last 25 years, making a fast and accurate quantized electron pump has proved extremely difficult. Here we demonstrate that the accuracy of a semiconductor quantum dot pump can be dramatically improved by using specially designed gate drive waveforms. Our pump can generate a current of up to 150 pA, corresponding to almost a billion electrons per second, with an experimentally demonstrated current accuracy better than 1.2 parts per million (p.p.m.) and strong evidence, based on fitting data to a model, that the true accuracy is approaching 0.01 p.p.m. This type of pump is a promising candidate for further development as a realization of the SI base unit ampere, following a redefinition of the ampere in terms of a fixed value of the elementary charge.
[Show abstract][Hide abstract] ABSTRACT: The upcoming 2011 general conference on weights and measures (CGPM) will consider a proposal to re-define the SI unit ampere by assigning a fixed value to the electron charge e, while simultaneously releasing the permeability of free space μ0 to become a measured parameter. This review provides the background to the proposed re-definition and describes research into electron pumps aimed at supporting it. Particular emphasis is given to recent results on semiconductor electron pumps, which are currently the most promising candidates for metrological current standards.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 01/2011
[Show abstract][Hide abstract] ABSTRACT: We study the effect of perpendicular magnetic fields on a single-electron
system with a strongly time-dependent electrostatic potential. Continuous
improvements to the current quantization in these electron pumps are revealed
by high-resolution measurements. Simulations show that the sensitivity of
tunnel rates to the barrier potential is enhanced, stabilizing particular
charge states. Nonadiabatic excitations are also suppressed due to a reduced
sensitivity of the Fock-Darwin states to electrostatic potential. The
combination of these effects leads to significantly more accurate current
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